Go to social or whatever section, scroll down to the list of topics, click on the one you want and on the upper left of your screen you’ll see an add topic button. Or go to My Account, you can do the same there.
I was wondering this as well. @Adirondackwannabe and @ibstubro‘s work if you happen to stumble upon a topic you are interested in, but what about if I want to add “kayaking” to my profile, but I don’t see any questions with “kayaking” tagged as a topic?